1. Technical Field
This invention relates to a method of selecting a wavelength for use in transmitting optical data along a path in an optical network.
2. Related Art
In an optical network, data is transmitted over optical fibres that interconnect network nodes (commonly called routers or switches). As data traverses the network it will traverse one or more network nodes. At each network node data is received on an input port, converted from an optical signal to an electrical signal, processed in the electrical domain, converted from an electrical to an optical signal and transmitted on the relevant output port. This conversion from an optical signal to an electrical signal and back again is called Optical-Electrical-Optical (OEO) conversion.
There are two main consequences of having OEO conversion at every node. Firstly, OEO conversion re-shapes, re-times and re-transmits the optical signal at every node meaning that each link in an end-to-end path can be considered in isolation and any fault or impairment on one link does not have a knock-on affect on subsequent links. Secondly, OEO conversion enables a node to receive an optical signal on a given wavelength and re-transmit it on a different wavelength, i.e. the nodes can perform wavelength conversion. However, OEO conversion is expensive to implement and acts as a bottleneck in the network because the maximum speed at which data can be switched from an input port to an output port depends on how fast the underlying electronics in each network node can operate.
In an all-optical network, data is also transmitted over optical fibres that interconnect network nodes. However at each network node, data is received on an input port, switched in the optical domain and transmitted on the relevant output port without any OEO conversion taking place. The lack of OEO conversion in all optical networking nodes (more commonly called optical cross connects (OXCs)) results in the cost of networking nodes being reduced. It is usual in an all optical network for several different wavelengths to be used simultaneously on each optical fibre by utilizing wavelength division multiplexing (WDM).
The lack of OEO conversion in an all-optical node means that nodes can no longer perform wavelength conversion so it is necessary to use the same wavelength along the entire end to end path. This is known as the wavelength continuity constraint and can result in a situation where an end-to-end path is not usable because despite the network having spare capacity (i.e. wavelengths that are not being used to transmit data), there is no one common wavelength available on all the links on the end-to-end path.
Current methods of selecting a wavelength for use in transmitting data involve considering all the wavelengths available for use on each link that actually constitutes the path and selecting one that is available end-to-end. In one such method, all the wavelengths on a link are sequentially numbered and the wavelength with the lowest number that is available end-to-end is chosen. A disadvantage associated with these methods is that they do not consider how a wavelength selection decision will affect future connection attempts along paths that have one or more links in common with the path for which the wavelength is being selected. As a result, a future connection attempt may be blocked because there are no wavelengths available end-to-end along the entire path that is to be set up.